1. Field of the Invention
The present invention is related to a method for forming a single crystal GaN substrate, and more particularly, to a method for forming a single crystal GaN semiconductor substrate, which allows fast homoepitaxial growth of GaN group materials, and formation of a bulk single crystal GaN substrate, which has a very high quality almost free from crystalline defects, of a size required for growth of thin film in fabrication of an optical device; and a GaN diode fabricated with the single crystal substrate.
2. Discussion of the Related Art
In a conventional growth of GaN which is used in fabrication of blue optical device, a sapphire(Al.sub.2 O.sub.3) substrate has been mostly used. However, the significant lattice mismatch (13.8%) and difference(25.5%) of thermal expansion coefficients between gallium nitride and sapphire cause difficulty in growing a good quality thin film. Even if a buffer layer disclosed in JP H4-170390 is employed in the growth of the thin film, the concentration of crystal defects occurring in the grown thin film is 10.sup.9 -10.sup.11 cm.sup.-2, which should be significantly reduced for practical laser diode applications, even though they may not cause any significant problems in LED applications. Although many efforts have been made for reducing the lattice mismatch employing silicon carbide(SiC) or spinnel as the substrate, which have lattice mismatches less than sapphire, the concentration of internal crystal defects in the grown thin film caused by the lattice mismatch could not be reduced significantly. Moreover, the difference of cleavage directions of GaN and sapphire in using a sapphire substrate causes difficulty in the application of a general cleaving method to fabrication of a laser cavity and also to the formation of electrodes on the back side of the sapphire substrate, which is an insulator. This places a limitation in the fabrication of a LED because the electrode formation process becomes complicated.
The requirement for employing a GaN substrate to overcome the aforementioned problems and to grow a large sized bulk single crystal GaN at room temperature equilibrium by an existing crystal growth method has been impossible because GaN has a melting point of over 2400.degree. C. while nitrogen N.sub.2 has an equilibrium vapor pressure of about 100 Kbar at 1,100.degree. C. and 10,000 Kbar at 1,500.degree. C. Recently, I. Gzegory et al. in "J. Phys. Chem. Solids, 56,656(1995)" discloses a solution method conducted at a high temperature and high pressure state of 1,300-1,600.degree. C. and 8-17 Kbar to obtain a thin single crystal plate of a several millimeter size having a good quality with crystal defect concentration of about 100/cm.sup.2, which is still not satisfactory for use as a substrate. Detchprhom et al. in "J. Crystal Growth, 123,384(1993)" discloses a method in which a thick GaN layer is grown by HVPE (Halide Vapor Phase Epitaxy) on a ZnO buffer layer which has a comparatively little lattice mismatch of 2.2% with GaN formed on a sapphire substrate and the ZnO buffer layer is removed to separate bulk GaN single crystal layer from the sapphire substrate, intending to use the bulk GaN single crystal layer as a substrate. However, this method has a limitation in obtaining a good quality substrate of a required size due to the unstable chemical etching of the ZnO layer. T. Okada et al. in "J. J. Applied Physics, 35(5), 1637, 1966" discloses a sublimation method in which powdered GaN is subjected to sublimation into a GaN substrate under an ambient of nitrogen and ammonia, which was found not satisfactory. And, recently, R. J. Molnar et al. in "MRS Symp. Proc. Vol. 423,221, 1996" discloses a method in which about 50 .mu.m thick GaN is grown on a sapphire substrate to form a homoepitaxy. However, as this method also requires a form of heterepitaxy, this method has a limitation in reducing the crystalline defect concentration in a GaN film.